add legend title to px.pie

[lucafs]

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.

Fixes #2966 issue and adds a new testing file to test changes asserting that the name in the parameter "names" is the same as fig.layout.legend.title.text in the fig variable.

Example

Code:

import plotly.express as px
df = px.data.gapminder().query("year == 2007").query("continent == 'Europe'")
df.loc[df['pop'] < 2.e6, 'country'] = 'Other countries' # Represent only large countries
fig = px.pie(df, values='pop', names='country', title='Population of European continent')
fig.show()

Results

Before:

After: